Because I've install gentoo on my External hardware, so I uninstall gentoo Portage for MacOSX.
But after I uninstall gentoo portage, the User named Portage always on my MacOSX login screen.
Is there anyway can let me delete this user ? I really dislike this inexistent user on my login screen ~
Thank you !

Since you've uninstalled portage, you should be able to simply remove this user from "/etc/passwd" as well its group "/etc/group".
Each user (each group respectively) gots a separate line in "/etc/passwd" ("/etc/group" respectively), so you've just to remove these line and the user (group) no longer exist on your system.
Regards,